The field of the invention is coherent imaging using vibratory energy, such as ultrasound and, in particular, ultrasound imaging of a fetus.
There are a number of modes in which ultrasound can be used to produce images of objects. The ultrasound transmitter may be placed on one side of the object and the sound transmitted through the object to the ultrasound receiver placed on the other side (xe2x80x9ctransmission modexe2x80x9d). With transmission mode methods, an image may be produced in which the brightness of each pixel is a function of the amplitude of the ultrasound that reaches the receiver (xe2x80x9cattenuationxe2x80x9d mode), or the brightness of each pixel is a function of the time required for the sound to reach the receiver (xe2x80x9ctime-of-flightxe2x80x9d or xe2x80x9cspeed of soundxe2x80x9d mode). In the alternative, the receiver may be positioned on the same side of the object as the transmitter and an image may be produced in which the brightness of each pixel is a function of the amplitude or time-of-flight of the ultrasound reflected from the object back to the receiver (xe2x80x9crefractionxe2x80x9d, xe2x80x9cbackscatterxe2x80x9d or xe2x80x9cechoxe2x80x9d mode).
There are a number of well known backscatter methods for acquiring ultrasound data. In the so-called xe2x80x9cA-scanxe2x80x9d method, an ultrasound pulse is directed into the object by the transducer and the amplitude of the reflected sound is recorded over a period of time. The amplitude of the echo signal is proportional to the scattering strength of the refractors in the object and the time delay is proportional to the range of the refractors from the transducer. In the so-called xe2x80x9cB-scanxe2x80x9d method, the transducer transmits a series of ultrasonic pulses as it is scanned across the object along a single axis of motion. The resulting echo signals are recorded as with the A-scan method and either their amplitude or time delay is used to modulate the brightness of pixels on a display. With the B-scan method, enough data are acquired from which an image of the refractors can be reconstructed.
In the so-called C-scan method, the transducer is scanned across a plane above the object as it produces a series of ultrasonic pulses. Only the echoes reflecting from the focal depth of the transducer are recorded. The sweep of the electron beam of a CRT display is synchronized to the scanning of the transducer so that the x and y coordinates of the transducer correspond to the x and y coordinates of the image.
Ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements sandwiched between a pair of electrodes. Such piezoelectric elements are typically constructed of lead zirconate titanate (PZT), polyvinylidene diflouride (PVDF), or PZT ceramic/polymer composite. The electrodes are connected to a voltage source, and when a voltage is applied, the piezoelectric elements change in size at a frequency corresponding to that of the applied voltage. When a voltage pulse is applied, the piezoelectric element emits an ultrasonic wave into the media to which it is coupled at the frequencies contained in the excitation pulse. Conversely, when an ultrasonic wave strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. Typically, the front of the element is covered with an acoustic matching layer that improves the coupling with the media in which the ultrasonic waves propagate. In addition, a backing material is disposed to the rear of the piezoelectric element to absorb ultrasonic waves that emerge from the back side of the element so that they do not interfere. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and 4,569,231.
When used for ultrasound imaging, the transducer typically has a number of piezoelectric elements arranged in an array and driven with separate voltages (apodizing). By controlling the time delay (or phase) and amplitude of the applied voltages, the ultrasonic pulses produced by the piezoelectric elements (transmission mode) combine to produce a net ultrasonic wave focused at a selected point. By controlling the time delay and amplitude of the applied voltages, this focal point can be moved in a plane to scan the subject.
The same principles apply when the transducer is employed to receive the reflected sound (receiver mode). That is, the voltages produced at the transducer elements in the array are summed together such that the net signal is indicative of the sound reflected from a single focal point in the subject. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal pulses from each transducer array element.
All these modes of ultrasonic imaging rely on the production of short pulses of ultrasonic energy. Each transmitted ultrasonic pulse and the resulting echo is a measurement which is repeated rapidly during a scan to acquire sufficient data for an image.
Obstetricians rely heavily on observations obtained by ultrasound imaging to study normal and abnormal fetal behaviors. Ultrasound is not audible to humans since it is typically in the 2 MHz to 10 MHz frequency range, well above the 20 kHz upper limit of human hearing. As a result, it is assumed that ultrasonic imaging is not disruptive and is a relatively benign imaging modality to employ when evaluating fetal behavior. Stimulation of the fetus is detrimental in at least two situations where ultrasonic imagers are used. Stimulation of the fetus may introduce errors in fetal evaluation tests, such as biophysical profile tests or fetal hearing evaluation studies. Fetal motions can also complicate the performance of delicate invasive procedures in which fetus immobility is critical.
The present invention arises from the discovery that pulsed ultrasonic imaging systems produce audible sounds which stimulate a fetus. A radiation force is produced by ultrasonic waves reflecting off an object and that force is modulated in amplitude at the pulse repetition rate of the ultrasound system. This produces audible sound that stimulates the fetus when it reaches a certain level. The present invention is an ultrasound imaging system which operates in such manner as to avoid fetal stimulation, and which signals the operator when operating conditions exist that may stimulate the fetus.
One aspect of the present invention is to provide an ultrasound imaging system in which the pulse repetition rate is outside the audible frequency range. This is achieved using either a continuous wave (CW) imaging method or a pulse repetition rate that is above audible frequencies.
Another aspect of the invention is to reduce the peak power of ultrasonic pulses such that any audible sounds produced by them will be below the level which stimulates the fetus. This may be achieved without significantly reducing the signal-to-noise ratio of the acquired image data by shaping each pulse to spread the power in time such that its peak power can be reduced. To maintain the resolution of the image acquired with such shaped pulses, a matched filter is used in the receiver to restore axial image resolution.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.